It is well known that galvanically isolated AC current sensing can be achieved with a voltage sense winding on a low permeability magnetic core around a conductor carrying the current to be sensed. In practice, the magnetic core need not completely encompass the current carrying conductor, but need only be of suitable shape and in the proximity of the conductor such that magnetic flux is induced in the core by the current to be sensed.
If the magnetic core permeability does not vary appreciably with magnetic flux density or frequency, the voltage induced in the sense winding is proportional to the time derivative of the sensed current. It is further known that the original current waveform can be reconstructed by integrating the voltage on the sense winding. If the relative magnitude of the flux induced in the core by the sensed conductor current is known, the integrator voltage can be scaled to accurately represent the quantitative current magnitude. If the relative flux magnitude is not known, a qualitative representation of the current waveform is still obtained.
Although the low permeability magnetic core referred to above is often one of several distributed-gap magnetic materials collectively referred to as "powdered iron", the sense winding may in fact be wound on a non-magnetic material with a relative permeability of unity. Alternatively, the sense winding may be wound on a high permeability core with one or more discrete "air gaps" in the magnetic material to create a well defined effective permeability.